cephadm.postinst uses hard coded path /home/cephadm/

Hello all,

### release ###
lsb_release -rd
#Description: Ubuntu 20.04.1 LTS
#Release: 20.04

### package version ###
apt-cache policy cephadm
#cephadm:
# Installed: 15.2.3-0ubuntu0.20.04.2
# Candidate: 15.2.3-0ubuntu0.20.04.2
# Version table:
# *** 15.2.3-0ubuntu0.20.04.2 500
# 500 http://de.archive.ubuntu.com/ubuntu focal-updates/universe amd64 Packages
# 100 /var/lib/dpkg/status
# 15.2.1-0ubuntu1 500
# 500 http://de.archive.ubuntu.com/ubuntu focal/universe amd64 Packages

### expected ###
/var/lib/dpkg/info/cephadm.postinst using $HOME from previously created new user 'cephadm' to prepare ˜cephadm/.ssh/authorized_keys

### what happened instead ###
/var/lib/dpkg/info/cephadm.postinst uses hard coded path /home/cephadm complaining with
  "mkdir: cannot create directory ‘/home/cephadm/.ssh’: No such file or directory"
if /etc/adduser.conf:DHOME was modified.

### Notes ###
While installing cephadm(8) we get
  "mkdir: cannot create directory ‘/home/cephadm/.ssh’: No such file or directory"
due to hard coded "/home/cephadm/.ssh" in /var/lib/dpkg/info/cephadm.postinst .
Previously /var/lib/dpkg/info/cephadm.postinst uses adduser(8), honoring
/etc/adduser.conf:DHOME - which may be changed from it's default "/home".

Modified /var/lib/dpkg/info/cephadm.postinst attached.

Thanks & best regards
Uwe